The art of ink-jet printing is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines have been implemented with ink-jet technology for producing printed media. The contributions of Hewlett-Packard Company to ink-jet technology are described, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985); Vol. 39, No. 5 (October 1988); Vol. 43, No. 4 (August 1992); Vol. 43, No. 6 (December 1992); and Vol. 45, No. 1 (February 1994); all incorporated herein by reference.
Generally an ink-jet image is formed when a precise pattern of dots is ejected from a drop-generating device known as a “printhead” onto a printing medium. Typically, an ink-jet printhead is supported on a movable carriage that traverses over the surface of the print medium and is controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to a pattern of pixels of the image being printed.
A typical Hewlett-Packard ink-jet printhead includes an array of precisely formed nozzles in an orifice plate that is attached to a thin film substrate that implements ink firing heater resistors and apparatus for enabling the resistors. The ink barrier layer defines ink channels including ink chambers disposed over associated ink firing resistors, and the nozzles in the orifice plate are aligned with associated ink chambers. Ink drop generator regions are formed by the ink chambers and portions of the thin film substrate the orifice plate that are adjacent the ink chambers.
The thin film substrate is typically comprised of a substrate such as silicon on which are formed various thin film layers that form thin film ink firing resistors, apparatus for enabling the resistors, and also interconnections to bonding pads that are provided for external electrical connections to the printhead. The thin film substrate more particularly includes a top thin film layer of tantalum disposed over the resistors as a thermomechanical passivation layer.
The ink barrier layer is typically a polymer material that is laminated as a dry film to the thin film substrate, and is designed to be photo-definable and both UV and thermally curable.
An example of the physical arrangement of the orifice plate, ink barrier layer, and thin film substrate is illustrated at page 44 of the Hewlett-Packard Journal of February 1994, cited above. Further examples of ink-jet printheads are set forth in commonly assigned U.S. Pat. No. 4,719,477 and U.S. Pat. No. 5,317,346, both of which are incorporated herein by reference.
Considerations with the foregoing ink-jet printhead architecture include delamination of the orifice plate from the ink barrier layer, and delamination of the ink barrier layer from the thin film substrate. Delamination principally occurs from environmental moisture and the ink itself which is in continual contact with the edges of the thin film substrate/barrier interface and the barrier/orifice plate interface in the drop generator regions.
While the barrier adhesion to tantalum (the adhesion occurring between the barrier layer and the native oxide layer which forms on the tantalum layer) has proven to be sufficient for printheads that are incorporated into disposable ink-jet cartridges, barrier adhesion to tantalum is not sufficiently robust for semi-permanent ink-jet printheads which are not replaced as frequently. Moreover, new developments in ink chemistry have resulted in formulations that more aggressively debond the interface between the thin film substrate and the barrier layer, as well as the interface between the barrier layer and the orifice plate.
In particular, a solvent, such as water, from the ink enters the thin film substrate/barrier interface and the barrier/orifice plate by penetration through the bulk of the barrier, penetration along the barrier, and in the case of a polymeric orifice plate by penetration through the bulk of the polymeric orifice plate, causing debonding of the interfaces through a chemical mechanism such as hydrolysis.
The problem with tantalum as a bonding surface is due to the fact that while the tantalum layer is pure tantalum when it is first formed in a sputtering apparatus, a tantalum oxide layer forms as soon as the tantalum layer is exposed to an oxygen containing atmosphere. The chemical bond between an oxide and a polymer film tends to be easily degraded by water, since the water forms a hydrogen bond with the oxide that competes with and replaces the original polymer to oxide bond, and thus ink formulations, particularly the more aggressive ones, debond an interface between a metal oxide and a polymer barrier.
Thus, it would be advantageous to provide an improved ink-jet printhead that with improved adhesion between the thin film substrate and the ink barrier layer.